Built-up bitumen felt roofs are the traditional flat roof system common to many
flat roofs in the United Kingdom.

What Are Felt Roofs?

Felt roofs consist of two or three layers of sheet felt materials that form a
dense impermeable barrier.
Roofing felt is a composite of an absorbent sheet of organic or synthetic fiber
that is impregnated/saturated and coated with a waterproofing agent such as tar
or asphaltic bitumen that prevents ingress of moisture.

The felt roof system is built up of multiple layers of high performance felts
and mastic asphalt over an insulated timber or concrete flat roof deck.
When applied the layers of vapour dispersion and control membranes are sealed
with graded roofing felts to guarantee a waterproof installation.

There
are several techniques for applying felt - depending on the materials used,
however felt is generally rolled out over the roof and bonded together by way
of hot bitumen or adhesives.Materials technology and new products have modernised the range of felts many
manufacturers supply, so modern man-made and composite materials felt roofs
that use modified bitumen; include rubber modified elastomeric bitumen and
polymer modified bitumen are now available.

Hot Works

Pour and Roll technique: where hot bitumen at 220-240°C is poured
in front of the felt as it is unrolled along the roof. The hot, sticky, viscous
bitumen is poured down the front of the roll to provide a continuous bead of
molten bitumen across the whole width of the felt. The bitumen fills voids and
when cold it provides a total contact adhesive bond to the layers and it seals
joints at the laps. Laps are never clean.

Torching-On:
torch on uses specially designed felt that is heated with a gas torch. No
bonding bitumen is required. The technique is generally used for small to
medium sized roofs; and flat roof repair where the new material must be fused
with the old to ensure total lap security or where access to the roof is
difficult. The application is unsuitable for direct bonding to timber and wood
based decks or lightweight polyester base materials.

Cold Works

Cold Applied Adhesive is applied evenly onto the roof surface and
the roofing felts are unrolled into position. Laps are sealed with the cold
adhesive.

Self-Adhesive
Membranes: Self-adhesive membranes have undersides that include a high-tack
adhesive with release paper. Once in the correct position on the roof, the
release paper is peeled off, and the membrane will stick to the base. They are
unsuitable for laying over uneven surfaces or in cold conditions. They require
pressure contact to assist full adhesion. For the DIY market, they are used for
houses and flat roof extensions, garages, garden buildings, workshops, sheds,
children's playhouses, dog kennels and rabbit hutches.

Surface Protection

Built up felt roofs require a protective surface to prevent damage
by UV radiation (solar) damage, human traffic and other impact.
Because asphalt is a natural product, it has a life span and the purpose of the
finish is to delay degradation.
UV-rays in sunlight would otherwise have the capacity to oxidize the roof
surface and cause the plasticisers to leach out causing the asphalt to become
brittle and more prone to blistering and cracking, which can cause water leaks.

Reflective
paints which require re-application every 3 to 4 years.
Roof balconies and terraces require the covering to protect the roof membrane
from the human traffic and pressures that could puncture of crack the surface
and break the waterproof seal.

Roof covering options include:

Tiles
such as porous concrete, fibre cement, plastic or rubber and bedded in an
adhesive